In the field of telephony, it has historically been considered to be too costly to provide high voltage signalling, such as ringing and coin telephone control, on a per-line basis. Instead, these functions have usually been provided by common generators. Each generator serves a number of lines, being switched onto each line when required, by a relay on the line circuit. However, this approach does have drawbacks, as will be described below, and does not easily extend to other forms of high voltage signalling, such as message waiting. As further requirements, to reduce impulse noise while ringing, the relay movement should be timed to coincide with a "zero crossing" of the ringing AC waveform. Also, party lines may require "selective ringing", involving: (a) providing the ringing voltage to one or other side of the line, relative to ground; (b) superimposing either a negative or a positive DC voltage onto the AC ringing voltage; (c) ringing at different frequencies. Due to the burden of providing the correct type of signal to each line as required, without overloading the generators when traffic is high, there can be significant time delay in providing signalling operations, so it is not possible to guarantee a desirable "immediate ring" feature. For message waiting signalling it is essential that the voltage be applied and removed slowly, in order to avoid sounding the subscriber's ringers. Also, the maximum current delivered to each line should be limited for safety reasons not to exceed a few milliamperes. These special requirements are additional to those providing ringing or coin operation, further increasing the cost.
Electronic techniques for supplying DC and AC power to a telephone line or other load line are well known. One reason for using electronic means for supplying DC power is to reduce heat dissipation within the line circuit. Heat dissipation can be further reduced by adjusting the voltage of the DC supply in accordance with the line resistance. Since line resistances differ, preferably this is done by providing efficient power conversion for each line individually.
U.S. Pat. No. 4,761,812 issued August 1988 (Hollis el al), teaches a constant power telephone line circuit. It comprises a flyback switching power converter having a switching transistor circuit, a transformer having a primary winding coupled to the switching transistor circuit and device for discharging energy stored in the core such that the power output is a constant power pulse. U.S. Pat. No. 4,674,119, issued June 16, 1987 (Chea, Jr.), describes a wide band high voltage amplifier for telephony. It also uses a flyback generator with a capacitor storage element coupled across the line and an error signal controls the energy transfer from the flyback converter to the line. U.S. Pat. No. 4,056,689, issued Nov. 1, 1977 (Freimanis), on the other hand, describes a communication line interface circuit including a floating current source supplying a constant current substantially independent of loop length. The line interface circuit is provided with circuitry for line supervision and the detection of dial pulse signalling.
A need therefore exists for an efficient power converter not only for the telephone system but also for systems where precise control of power flow into and out of a load is required. For example, such converters are useful in the field of motor control of computer tape drives, and of read/write heads in computer disk drives, which are required to have rapid starting, stopping, and reversing characteristics.
In telephony, the converter should be able to supply DC power of an adjustable voltage for providing balanced line feed for the idle, addressing and transmission states of the line. It should also be able to supply AC power of a desired waveform at voltages higher than the DC source voltage. A voltage higher than a DC source is used if, for example, it should happen that the DC source voltage typically from a battery is unusually low. A voltage lower than the DC source might be used when the line resistance is less than the normal maximum. The converter can alternatively supply signalling power to one or the other side, or both sides, of the line, when the situation requires voltages higher than or of the opposite polarity from that of the DC source. These situations include ringing, coin telephone signalling and control, and message-waiting signalling.